Nov. 21, 2011 -- Being overweight in middle age is now recognized as a risk factor for Alzheimer's disease, but being overweight or obese later in life is associated with a lower risk for age-related memory decline.

New research may help explain this, suggesting that weight loss may be one of several bodily changes that occur in Alzheimer's disease long before memory problems and other symptoms appear.

Among older people who had no memory problems or who showed evidence of mild mental impairment, being a normal weight or underweight -- defined for this study as having a body mass index below 25 -- was associated with having more indicators of a high risk for Alzheimer's disease. Body mass index is a way to measure body fat based on a ratio of weight and height.

The association was strongest in people with the least evidence of memory loss, suggesting that lower weight was not related to forgetting to eat and other behavioral changes associated with a failing memory.

"Our research suggests that there are very early [bodily] changes associated with this disease," says researcher Jeffrey M. Burns, MD, who is the associate director of the University of Kansas Alzheimer's Disease Center. "We think of Alzheimer's as just a disease of the brain, but this may not be the case."

Alzheimer's and BMI

In earlier studies, Burns and colleagues found evidence that decreases in bone density and muscle mass are also related to early mental decline.

Their latest research, published in the journal Neurology, included 506 people with no memory problems, those with mild mental impairment, and people with Alzheimer's disease.

Brain imaging was conducted to look for indicators associated with Alzheimer's disease.

Study participants with no evidence of memory or thinking impairments and those with mild impairment who showed evidence of the Alzheimer's indicators were more likely to be a normal weight or underweight than those who did not show evidence of the indicators.

Specifically, brain imaging revealed that 85% of people with mild mental impairment who were normal weight or underweight showed signs of what are called beta-amyloid plaques, which are a key trait of Alzheimer's disease.

Among people with mild impairment who were overweight, only 48% showed signs of the plaques.

Lower Weight May Be Early Sign

A similar relationship was seen in people with no recognized memory or thinking impairments.

It was not clear if the normal and underweight people in the study had lost weight or if their weight had been stable. And because there are many reasons why people lose weight as they age, the association may not help doctors identify people at risk for Alzheimer's disease.

But Burns says the findings may have important implications for researchers studying Alzheimer's disease.

"We've had this paradox that midlife obesity is a risk factor for developing Alzheimer's disease, but people with dementia are more likely to be underweight," the Alzheimer's researcher tells WebMD. "The recognition that these people have lower BMIs long before they develop symptoms related to Alzheimer's does suggest that low weight may be a very early [sign] of this disease."